Carburized thoriated tungsten electrode and method of enhancing its emissivity



W. E. HARBAUGH a Jan. 14, 1958 v 2,819,991

CARBURIZED THORIATED TUNGSTEN ELECTRODE AND METHOD OF ENHANCING ITSEMISSIVITY Filed Aug. 30, 1952 a? /,I V

M1115 il fiz'i i gh M United States Patent *9 "ice CARBURIZED THORIATEDTUNGSTEN ELEC- TRODE AND METHOD OF ENHANCING ITS EMISSIVITY Willis E.Harbaugh, Bareville, Pa., assignor to Radio Corporation of America, acorporation of Delaware Application August 30, 1952, Serial No. 307,295

9 Claims. (Cl. 148--13.1)

The present invention relates to improvements in socalled carburizedelectrodes for use in electron tubes, and to improvements in the art ofmaking the same.

Under the prior art practice certain power tube electrodes have beencarburized in order to enhance their thermionic emission and alsobecause the carbon deposited on the electrodes provides a continuinggettering. action during operation of the tube. However, in themanufacture of power electron tubes in which carburized thoriatedelectrodes are used, difiiculty has been met in obtaining carburizedelectrodes which have uniformly high electron emission capabilitiesimmediately following tube processing or activation. Also, carburizedelectrodes made according to prior art practices often have a tendencyto buckle or camber. Such buckling or cambering results in changes inthe electrical characteristics of the tube in which the electrodes areused and may, in extreme cases, cause short circuits between adjacentelectrodes.

A principal object of the present invention is to pro vide improvedcarburized thcriated electrodes which obviate the above mentioneddisadvantages.

Another object of the present invention is to provide a commerciallypractical process for manufacturing thoriated electrodes having improvedand stable electron emission.

Still another object of the present invention is to provide means forachieving a high degree of mechanical accuracy in the manufacture ofelectrodes for use in high power electron tubes.

A further object of the present invention is the provision of a processfor stretching carburized thoriated tungsten electrodes which insuresthe formation of the desired crystalline phase and structure in thecarbide layer on the electrode.

Stated generally, the foregoing and related objects are achieved inaccordance with the method of the invention by stretching a carburizedthoriated electrode beyond its elastic limit under controlled conditionsof temperature and tension.

In a preferred embodiment, electrodes which have been pre-carburized inaccordance with the process disclosed and claimed in my copendingapplication, Serial No. 165,093, filed May 29, 1950, entitled-Process ofCarburizing, now abandoned, are then heated to approximately 1600 C. Atthis temperature each of the electrodes is placed under axial tensionand stretched beyond its elastic limit until the electrode ispermanently elongated about one-half of one percent of its total length.

Cathode electrodes so stretched have been found to have high electronemission capabilities immediately after activation of the tube in whichthey are used. Also, in accordance with the present invention electrodesso stretched have been found to be uniformly straight and free fromtendencies to buckle and camber.

The invention is described in greater detail in connec- 2,819,991Patented Jan. 14, 19 58 2 tion with the accompanying single sheet ofdrawings, wherein:

Fig. l is an elevation view, partly in section, of an apparatus forcarrying out the method of the invention; Fig. 2 is a fragmentaryperspective view of a carburized electrode in which part of thecarburized layer has been removed;

Fig. 3 is a greatly magnified view of a small area on that. part of thecarburized electrode of Fig. 2 which hashad a part of carbide layerremoved; Fig. 4 is a greatly magnified view of the appearanc of the areaof Fig. 3 after the electrode of Fig. 2 has been stretched, inaccordance with the invention. Fig. 1 shows one form of apparatussuitable for precarburizing and stretching a thoriated tungstenelectrode 10 prior to mounting it in an electron tube. The electrodeIll, which is to be carburized, comprises a rod-like solid core ofthoriated tungsten which, in the instant case, is trapezoidal incross-section (see Fig. 2) throughout a greater portion of its lengthand terminates in oppositely located end sections 11 of reduced diameterand consequent reduced heat-conducting properties. The end-sections 11each terminate in a mounting head or hook 12 by means of which theelectrode is removably supported upon two oppositely located slotted andgrooved arms 13, 14 which are each supported from electricallyconductive members 15, 16 in any suitable manner. Members 15 and 16 areconnected to the terminals of a source of electrical power (not shown)by leads 17, 13. Member 15 is insulated from standard 19 and supportedthereon. Member 16 is slidably connected to standard 19 in such manneras to permit axial elongation of electrode 10 when heated.

Cooling means, in the form of tubes 2%) direct dry hydrogen, or othercoolant which is inert with respect to thoriated tungsten, from a source(not shown) upon the reduced end portions 11 of the electrode It tomaintain said electrode ends at a temperature sufiiciently low' toprevent their burning out or becoming carburized.

A bell 21 is movably mounted over, and may be lowered around, standard19 and the parts supported thereon with its lower end 22 open to theatmosphere if a light gas is used; with a heavy gas, filling from thebottom would be necessary. Tubing 29 opens into the bell 21 and isconnected to controlled sources of gases which are to be introduced intothe bell to form an atmosphere most favorable to carburizing.

The apparatus of Fig. l, as described above, is similar to the apparatusused in my co-pending application, Serial No. 165,093, previouslymentioned.

In order to apply axial tension to the electrodes in applied throughslideablc member 16, on the electrode:

19. Extension rod 28 which is attached to slideable member 16, actuatesa dial indicator 26 that is attached to the supporting frame 27. Thedial 26 indicates .the-

amount of elongation or stretching of electrode 10.

Briefly, the electrode carburizing process described in my co-pendingapplication, Serial No. 165,093, previous ly mentioned, is as follows:

(1) The electrode to be carburized is heated to (2) The electrode ismaintained at the above tempera-- ture while a mixture of hydrogen pluscarbon bearing K. in hydrogen to recrystallize the tungsten;

vapor is formed. This is continued for approximatelyv one minute until athin layer of massive type carbide is formed;

(3) The temperature is increased to 2800 K.; and the carbon bearingvapor is removed, so that the cathode is heated very hot in a hydrogenatmosphere. This causes the carbide to penetrate deeper into thetungsten and form the laminated type of carbide desired.

By way of explanation, the laminated phase of tungsten carbide occursafter the crystalline structure of the tungsten carbide has passedthrough the massive phases WC and W C. In the laminated phase the carbonconcentration by weight percent varies from 3.16 to 2.45. Laminatedphase tungsten carbide is discussed in relation to its uses in electronemitting electrodes in Carbide structures in carburized thoriatedtungsten filaments, C. W. Horsting, Journal of Applied Physics, vol.1.8, No. 1, p. 95, January 1947.

The following step, that of stretching the electrodes, is the essence ofthe present invention:

The temperature of the electrode is reduced to approximately 1600 C. andaxial tension is applied between the ends of the electrode in order tostretch the electrode beyond its elastic limit until it is permanentlyelongated about one-half of one percent of its total length.

It should be emphasized that the carburizing steps and the stretchingstep need not be one continuous operation.

As a specific example of how the present invention is accomplished,including the carburizing steps, thoriated tungsten electrodes 10 aremounted between upper and lower supports 13, 14, as previously describedin connection with Fig. 1. The electrodes 10, of substantiallytrapezoidal cross section, have a width of .057 inch narrowing to .040inch with the narrow face rounded. The thickness from front to rear is.037 inch. The electrode is approximately 8.6 inches long. The reducedportions 11 are cooled by jets of pure dry hydrogen which pass throughthe tubes 20. Bell 21, which is 24 inches high and 14 inches indiameter, is lowered and the electrode 10 is electrically heated (bymeans of current introduced through leads 17, 18) for thirty seconds at2600 K. in an atmosphere of pure dry hydrogen which is continuouslyflushed through the bell at the rate of 40 cubic feet per hour; the end22 of the bell being open to the atmosphere. This heat treatment, thoughbeneficial, is not essential. Then, while maintaining the electrode atthe temperature of 2600 K., carbon bearing vapor is admitted by adding aflow of 12 cubic feet per hour of hydrogen carrying benzene vapor to the40 cubic feet per hour of pure hydrogen which was flowing through thebell. The electrode is then maintained at a temperature of 2600 K. for aperiod of one minute.

The benzolated hydrogen is formed by passing hydro gen at the rate of 12cubic feet per hour first through a dryer and then through an atmosphereof benzene vapor in a carburetor. The carburetor has an internaldiameter of 4 /2 inches with a pool of benzene at room temperature. Thehydrogen inlet and outlet are both .200 inch internal diameter tubingwith their openings inches above the surface of the benzene. The wallsof the carburetor extend four inches above the surface of the benzeneand then taper inward at an angle of about 45 degrees.

The source of hydrocarbon is then cut off; the flow of the hydrogen ismaintained; and the electrode is simultaneously raised to a temperatureof 2800 K. and held there for two minutes.

Following this, the electrode temperature is reduced to approximately1600 C. and axial tension applied via the lever and fulcrum arrangement24, 25 until the electrode is stretched one half of one percent, or .040inch. In order to achieve this elongation a force of approximately25,000 pounds per square inch is required.

It is to be understood that substantial variations in the crosssectional thickness of the electrode 10 being carburized may require orpermit corresponding changes in the duration of the heating times, withthe temperature being held constant. However, these variations may bereadily determined.

Electrodes stretched in the above described manner undergo a physicalchange of structure which is identifiable under microscope examination.

Referring to Fig. 2, there is shown a fragmentary perspective view of aportion of an electrode 31 having a carbide layer 32, a portion of whichlayer has been removed from an area 33 adjacent the end of electrodeportion 31.

Fig. 3 is a greatly magnified view of the appearance of the area 33, asseen under a microscope, if the electrode of which it is a part has notbeen stretched in accordance with the present invention.

Fig. 4, however, represents a greatly magnified view of the appearance,as seen under a microscope, of area 33 when that area has been stretchedin accordance with the present invention. The arrow 39 indicates thedirection in which axial tension was applied.

The area 33 of Fig. 3 shows no particular pattern of crystaldistribution, yet the area 33 of the stretched electrode, shown in Fig.4, shows separation lines 34 or striations (the dark streaks) runningapproximately parallel to the axis of the tension applied to theelectrode. These striations may be attributed to the stretchingoperation.

Fig. 4 is merely illustrative of the physical appearance of a portion ofthe stretched electrode, and it cannot be determined that the striationsthemselves are the cause of the improved electron emission of theelectrodes.

One possible explanation for the enhanced emission of stretchedelectrodes is that the increase in emission is caused by a slightseparation between grain boundaries of the carbide layer so that morethorium is free to diffuse to the surface. Another possible explanationis that stretching causes the crystal structure to be re-oriented insuch a manner as to present more emissive faces to the surface of theelement. It has been found by other investigators that in a singlecrystal certain faces have better electron emitting properties.

The advantage of straightening the electrodes by stretching may beapplied to electrodes other than cathodes. For instance, grid strandsfor tubes of the type shown and described in U. S. Patent 2,544,664,issued March 13, 1951, to Garner, Parker and Harbaugh may bestraightened by this stretching process. Approximately the sametemperature and tension is used for stretching such tungsten gridstrands as is used in stretching the heretofore described carburizedthoriated tungsten cathode strands. The temperature is approximately1600" C.; and tension is applied until the strand is elongatedapproximately one-half of one percent.

What is claimed is:

1. The method of making a thoriated tungsten filamentary cathodeelectrode, said method comprising the steps of carburizing saidelectrode, heating said electrode to approximately 1600 C., and applyingaxial tension to stretch said electrode beyond its elastic limitapproximately one-half of one percent of its length.

2. The method of enhancing the electron emissivity of a thoriatedtungsten thermionic cathode which is carburized in the laminated phase,said method comprising heating said cathode to approximately sixteenhundred degrees Centigrade and stretching said cathode beyond itselastic limit approximately one-half of one percent of its length.

3. The method of enhancing the electron emissivity of a carburizedthoriated tungsten thermionic cathode element, said method comprisingheating said element to approximately sixteen hundred degrees centigradeand stretching said element beyond its elastic limit about onehalf ofone percent of its length.

4. The method of making an elongated filamentary cathode, said methodcomprising the steps of carburizing a thoriated tungsten electrodeelement, and heating said element to approximately 1600' C. whilesimultaneously applying sufficient axial tension to stretch said elementpast its elastic limit about one half of one percent of its length toproduce microscopic surface striations which extend substantiallyparallel to the longitudinal axis of said element.

5. The method of making a carburized thoriated tungsten electrode, saidmethod comprising the steps of carburizing said thoriated tungstenelectrode to produce on its surface a layer of laminated phase carbide,heating said carburized electrode to approximately sixteen hundreddegrees centigrade, while simultaneously applying sufiicient axialtension between the ends of said electrode to stretch said electrodebeyond its elastic limit approximately one-half of one percent of itslength.

6. The process of making a carburized thoriated tungsten element,comprising heating said thoriated tungsten element in an inertatmosphere at a predetermined temperature and stabilizing thecrystalline structure, then while continuing to heat said element atsaid temperature, adding carbon bearing vapor to said atmosphere tocreate and maintain a carburizing atmosphere, said temperature beingsufiiciently high to decompose the carbon bearing vapor, discontinuingthe addition of hydrocarbon vapor and continuing to heat said element ata substantially higher temperature while the carbon bearing vaporcontent of the atmosphere is rapidly being depleted, reducing thetemperature of said element to approximately 1600 C. and applying axialtension between its ends to stretch said element beyond its elasticlimit approximately onehalf of one percent.

7. The method of carburizing a thoriated tungsten element, comprisingheating said thoriated tungsten element at a predetermined temperaturein a carburizing atmosphere to which carbon bearing vapor is added tomaintain the carbon concentration and forming a layer on said elementwhich is substantially the massive phase W C, said temperature beinghigh enough to decompose said vapor, then reducing the addition ofcarbon bearing vapor and continuing to heat said element in saidatmosphere in which the carbon content is rapidly reduced, substantiallyall of said W C being converted to the laminated phase, then reducingthe temperature of said element to approximately 1600 C. and stretchingsaid element beyond its elastic limit approximately one-half of onepercent of its length.

8. The process of carburizing a thoriated tungsten element, comprisingheating the thoriated tungsten element in a carburizing atmospherecontaining an inert gas and hydrocarbon vapor at sufiiciently hightemperature to decompose the hydrocarbon, then continuing the heating ofthe thoriated tungsten element at a higher temperature in an atmospherein which the hydrocarbon content is rapid- 1y decreased, reducing thetemperature of the element to approximately 1600 C. and applying axialtension between the ends of said element to stretch said element beyondits elastic limit approximately one-half of one percent of its length.

9. An elongated carburi'zed thoriated tungsten thermionic cathodeelement which has been heated to approximately 1600 C. and stretchedbeyond its elastic limit about one half of one percent of its length.

References Cited in the file of this patent UNITED STATES PATENTS2,246,162 Benjamin June 17, 1941 2,294,389 Dean et a1 Sept. 2, 19422,310,094 Kroll Feb. 2, 1943 2,400,893 Thurber et a1. May 28, 19462,406,966 Pfeil Sept. 3, 1946 2,450,007 Litton Sept. 28, 1948 2,521,663Zunick Sept. 5, 1950

1. THE METHOD OF MAKING A THORIATED TUNGSTEN FILAMENTARY CATHODEELECTRODE, SAID METHOD COMPRISING THE STEPS OF CARBURIZING SAIDELECTRODE, HEATING SAID ELECTRODE TO APPROXIMATELY 1600*C., AND APPLYINGAXIAL TENSION TO STRETCH SAID ELECTRODE BEYOND ITS ELASTIC LIMITAPPROXIMATELY ONE-HALF OF ONE PERCENT OF ITS LENGTH.